Don't know about the Garonne but the Lateral a La Loire is part of the north / south Route Bourbonnais linking the Seine to the Saone and Rhone and in 2010 / 2012 there was 'some' commercial traffic. But if push comes to shove is there enough commercial stuff these days to justify the maintenance spend? Don't know. There used to be a fair amount of hotel boat traffic down the southern end, the Canal du Centre.
The other north south routes, which hopefully they'll keep maintaining are the Champagne Bourgogne down the centre and the Canal des Vosges in the east (formerly Canal de L'est) though that seems to struggle for water. For example the branch to Epinal seems to be closed regularly.
The Ardennes and Ainse, further north and east / west canals, were choked with weed in places back in 2014 but very rural and pretty.
I suspect we've seen the best of both French and UK waterways, funding for both is a huge issue with the financial hole we're in.
Similar now I suspect but back in 2008 Dutch canals, used extensively for commercial traffic, were both well maintained and licence free (though both they and Belgium are hot on boat safety).